Sensors for the measurement of position and displacement may be of the multiple-coil inductive sensor type including the differential transformer and its variations, both linear and rotary, and also synchronous AC generators adapted to serve as variable transformers in the measurement of angular position.
The linear variable differential transformer (LVDT) uses three windings and a moveable core to sense linear displacement. A primary winding, two secondary windings, and a moveable core are energized at the primary with an alternating current. The two secondary windings are connected in series opposition, so that the transformer output is the difference of the two secondary voltages. When the core is centered, the two secondary voltages are equal and the transformer output is zero. This is the balance or null position. When the core is displaced from the null point, the two secondary voltages are no longer equal in magnitude and the transformer produces an output voltage. Motion of the core in the opposite direction produces a similar effect with 180.degree. phase reversal of the alternating output voltage, i.e., the phase angle is positive (no phase shift with respect to the excitation) or negative (180.degree. phase shift with respect to the excitation) depending on which side of null the core is positioned. A demodulator circuit can be used to produce a DC output from this winding configuration. Differential transformers are also available in a rotary version for angular measurement in which the core rotates about a fixed axis. A product detector is normally used for sensing phase reversal when passing through the null point.
Other winding configurations are used in synchros, resolvers and microsyns.
The construction of a synchro is similar to that of a miniature three-phase synchronous motor or generator. The stator contains a three-phase winding and the rotor is excited with a constant single phase AC voltage while the shaft moves at low speeds or stays stationary. Basically the synchro is a transformer with one primary (the rotor) and three-secondaries (the Y-connected windings of the stator). The voltages induced in the secondary windings are proportional to the cosines of the angles between each stator coil and the rotor.
A resolver synchro is similar to a synchro generator in construction, but the stator contains only two windings oriented at 90.degree. relative to each other, and they are employed to resolve rotor position into sine and cosine component voltage signals.
The various types of displacement sensors may be used in computing servomechanisms and other electromechanical computers. When used in digital computers, it is necessary to convert the analog signal information into digital words for use by the signal processor. Usually, a rectification process is utilized to convert time-varying secondary output signals, having a zero average, into a rectified signal having a DC average value. Unfortunately, this process destroys the phase information contained in the secondary output signals. Unless the information is extracted before rectification and later converted to useful digital information as well, the displacement sensor will necessarily be restricted to operation in a limited range. Thus, an LVDT would be restricted to use on one or the other side of the null point while a synchro resolver would be restricted to operation in one quadrant only.
It would be useful to have a simple and inexpensive means of obtaining this phase information.